Electrodynamic loud speaker



May 28, 1935. L. THOMPSON I ELCTRODYNAMIC LOU`D ASPEAKER Filed July 50, 1952 AMPLIFIER c INVENTR Lincoln Tho Patented May 28, 1935 Y l ELo'rnoDYNAMIoLoUD SPEAKER Lincoln Thompson, Naugatuck, Conn. Application July 30 isaseriaiNo 626,416

Y1.2 Claims. This' invention relates. to devices for translating electrical oscillations -into mechanical.-

nature, in which the inductance and total im pedance of the audio frequency circuitand the mechanical inertia of the moving element Will be .5 of very smallmagnitudes, whereby the transmission of oscillations will be carried out with a high degree of Yfldelityand efficiency. I A further object is to provide a device of the above nature in whichinactive material is excluded from the airgap, whereby a1 mostefficient use will loe-made of the'conducting material, of

the audio frequency circuit. A further object is to provide a device of the above nature in which the vibrating conductor and associated leads Vare adapted-to be stamped out of a single blank, giving a uniform and inexpensive product. f a

vAfu'rther object isfto provide Aa device ofthe above nature which will be simple and rugged in 30 construction, inexpensive to fabricate andassemble, light in weight, ornamental appearance, and very dependable and durable in service.

. In the drawing: i

Fig. 1 represents a sectional view of an electro- 35 dynamic acoustic vibrator embodying the invention.

Fig. 2 isaperspectiv'e'front View of the transformer and vibrating element, as itappears with the loud speaker cone detached therefrom.

40 Fig. 3 is a sectional rear view of the vibrating unit with the transformer and core plate omitted.-

Fig'. 4 is a sectional view of a modified formrof the connector between the single coil vtransformer secondary and the lead strips for conducting -the 45 current to the movable' single coilvibrating ring.l

Fig. 5 is a sectional View of a modification. Y Previous Vcommercially successful electromechanicaldevices of the above nature have been of three :general types, the electromagnetic, theV flux, thus introducing into the 'circuit thegprop.-Y

:erty of inductancawhilethe electrostatic type is 5.5,- essentially a condenser (one Opf-.whose plates is vibratory), having the property of capacitance. t is a well known fact that the total impedance of any circuit having reactance, either in the form of'zinductance or capacitance, will vary with the value of tl'ieappliecl'l frequency, and that therefore the response of such a circuit to a voltage of varying :frequency will not faithfully represent the variations. in value ,of the-voltage.

, It is further well known that certain types of vacuum-tube amplifiers used in conjunction with radio loudspeakers function efficiently over only a small range of impedance load values in their output circuits, so that it is difficult to obtain uniform performance ofthe lamplifier over the wide band of frequencies desirablefor the faithful reproduction of speech or music.

' From the above it will be'seen that it is doubly important for the impedance of the audio frequency circuit of such devices to maintain a substantiallypconstant value throughout the Whole range of vfrequencies to which it must respond. Since the total impedance value of an electrical YPATNT OFFICE.

circuit represents a combination ,of resistance andV reactance valuesan'd since 'the resistance. component does not vary with frequency, it follows that by having-a relatively low value of `ratio of reactance to resistance, Lthe Vvariation of impedance with -frequency will be minimized.:` For this reason Vthe practice of deliberately introduc` ing lresistancel into the circuit is sometimesresorted to, butonly at a material "sacrificeof eiii-v ciency in the translation. I f

By means of this invention the' total amount" ofV inductive reactance 'inherent tothe electro-` dynamic movement has been greatly reduced, Withthe resulting production of a device whose impedance 'variation with.`frequency is much lower than with previous devices of this character. v v In loud speakers'in general, where use is made of a Wound coil of a large number of turns, it is, of course, essential that a certain portion of the spaceV withink the air gap be occupied by inactive material inthe form'ofinsulation,y Whereas inthe type embodied in this invention, the vibrating element, consisting in a'single closed ring of metal, requires no insulation Within'the air gap, allowing the space to be used with a maximum of eiciency, and incidentally permits of a very rugged and'inexpensive' type of construction.v i Referring now to 'the drawing, in which'` like reference numerals denote corresponding parts throughout the several Views, the Vnumeral. Il. (Figs. 1 and 3), indicatesl an iron yoke having pole 2 y y pieces I2 and I3, with cyundricauy curved 0pposing faces. symmetrically positioned therebetween is a cylindrical iron core I4, leaving a pair of annular Vsubstzyintially semicircular air gaps I5 and I6 between the respective pole pieces and the iron core. The core I4 is rmly xed in position relative to the yoke by a plate I1 of nonmagnetic material, secured to the rear surface of the core Yby screws I8 and to the rear surface of the pole pieces. I2 .and I3 by screws (not shown) threadedly engagedwith tapped holes I9 in said pole pieces. The yoke II, with its pole pieces I2 and I3, the core I4, and theair gaps I5 and I I6, together constitute a magnetic circuit, adapted to be excited by two magnetiaing coils'Z andA 2|, energized by a battery Zia` or other direct current source. rEhe coils2 and 2l are interconnected in such a manner that their polarities are additive about the magnetic circuit,'producing a eld similar in its distribution throughout the iron parts and the Aair gap to that rof the'well known bipolar dynamo-electric machine.

symmetrically Vdisposed in the Vannular space provided by the 'twoair gaps I5 and I Bis a vibrating element 22 formed in the shape of a continuous ring of highly conductive non-magnetic material, such as copper,v aluminum, silver or the like, said ring 22 having integrally formed thereon a plurality of upturned lugs 23,*adapted to be attached to a cone diaphragm-(brother Sound-radiating member. lSupporting the vibrating ring 22, and'integrally attached'thereto are a pair of bifurcated curved flexible conducting strips 25 and 26, vadapted to conduct electrical currents to and from the vibrating ring 22 at diametrically opposite-points. n 5

`The dimensions and curvature of the strips 25 and 2B are such as to allow the ring 22 .to vibrate freely in an axialr direction withinits range of vibration. The ring 22 and flexible strips V25 and 25 arepreferably formedfrom a single piece of material, makingfor simplicity'oi' production and uniformityof quality. YFor the purpose of furnishing arigid support forthe flexibleconducting strips 25 and 26,'and thus maintaining the vibrating ring 22 concen-VA trically withinthe air gaps I5 and I6, theV ends of`s'a'ld strips are secured to bent-over transverse portions 21 and 28 of apair of T-.shaped angular conducting plates 21a 'and 28a, asindicated in through a centrally located rectangular recess in the core I4, and has an integral ilange portion 32, adapted to t'into a recess in the forward face of saidr core, and to be secured therein byl screws 33.

- The outwardly-entending bent-over'transverse portions 2'1 and 28 of the T-shaped plates 21a and 28a` lie in a plane parallel to that of the front surface of the core I4, andare drilled anel.tapped at their outer ends, forming abutments adapted tending to relieve twisting strains upon the metal of members 25 and 25 when being assembled, and

also to improvethe electrical contact between thev abutments and the conducting members. Y

It will be seen that there is thus provided a rigid mechanical structure adapted to furnish a secure support for the strips 25 and 26 connected to the vibrating ring 22, thus maintaining said ring concentrically within the air gaps I5 and I6, but permitting'it to vibrate axially within the range of its travel.

As a means of transmitting the mechanical vibrations of the ring 22 t0 the ambient air in the form of sound waves, provision is made of a conical diaphragm 36 .of the conventional type, formed of parchment or the like, and cemented or otherwise secured at its inner end to the lugs 23 of the vibrating element 22. rEhe outer end of the diaphragm 36 is resiliently supported by a ring 3l of thin leather or similar material, held between a pair of circular clamping plates 38 and 39 by means of bolts 40. The inner plate 3E! is rigidly attached tothe main rleld structure by a pluralnity of brackets, one ofwhich is indicated in Fig. 2

, to the-rear ends of the plates 21a and 28a as by screws 41 threadedly engagedwith the same but terminating short of the insulating sheet 29. Underv someconditions, the joint between the secondary coil Ylill rand the .conducting plates 21a and 28a may be eliminated, and the parallel conducting plates 2Ia` and 28a may be formed integralwiththe secondary coil44 as shown in Fig. 4, thus simplifying the construction and some-A what reducing the electrical resistance of the circuit. Y y p Operation j When the field coils 2U and 2| are energized with direct current from the battery 2 la the field structure will become magnetized inV a manner similar to'that of a bipolar dynamo, the magnetic flux entering the core I4 from one of the two pole, pieces I2 and I3, emerging fromvthe core toward the `other pole piece, and completingits4 circuit through the yoke I I. Y

It-will be understood that the amplied alternating'or oscillatory currents derived either from a microphone, a phonograph pickup, or a radio system etc. will pass through the primary Winding 43 of lthe transformer 42 and induce the secondary winding 44 electromotive forces adapted to produce in the electrical circuit formed by the plates 2'Iaand28a, the flexible members 25 and 26, and the vibrating element 22, currents of similar wave-form but of much greater magnitude than the original currents.

The relative directions of currentrow in the electrical circuit at a certainl selected instant are through the'curved4 strips of the bifurcated flex-Y ible memberf25 to their junction point Jwhere l connection "is made withv the annular vibrating ringf22. Atthis f-point the two portions'of curminimum value.

the iiexible conducting strips 25 and l2li arewell Y spaced apart and kept as close as possible to the j return paths of the currents through the Vvibrating ring 22, a similar result is produced. VBy

means rof this construction,` it will be seen thatV the inductive effect ofthe interconnecting portions of the circuit will be maintained at a minimum value.v It will be observed',moreover, Athat the Vcurrent in the 'vibrating ring 22, instead of owing completely around the core I4,` (an arrangement which would vproduce a loop,circuit around a mass of iron, and thus make forhigh` inductance) divides into two parts owing ,in

" opposite directions about the Vcircunfiference of the core, an arrangement which reduces the` inductive eiect to the Y f f 1 By means of Vthis construction'it will be seen that not only will .the resistance of the circuit I be kept low, by the provision of ample crosssection and short paths for the current, but in addition by theavoidance of loops either with or without included magnetic material, the inductanceof the circuit will be maintained at a It therefore follows that the tendency of the impedance of the circuit to vary with impressed frequency'is minimized,v and currents owing in the vibrating elementf22 will tend faithfully to represent the voltage applied to the amplier. Y

From the well-known principle of the electric motor it will be seen, that the tendency of current flowing in the vibrating ring 22, as it lies in the strong magnetic eld in the air gaps, willV be to simultaneously impart to each side of the ring a thrust, perpendicular to the directions of both the ield flux and the current, or axially of the ring. Moreover, it will be observed that the direction of thrust on each side of the ring in the air gaps l5 and I6 will be the same along its axis, so that when receivingV current of one po -larity from the transformer, the vibrating ele,

ment will be urged forward, while when the current of the opposite polarity is received it-will be urged backward. The mechanical vibrations of the ring 22 will thus be caused to faithfully follow the oscillationsof the current owing in the windings of the transformer 42, theexible members 25 and 26 bendingv suiciently to permitV the vibratoryy motion without allowing any other displacement of the vibrating element. i

It will be understood that the conical dia-f phragm 36, rigidly fixed to the lvibrating element, will partake of its vibratory motion, and impart this motion in greater volume to the surrounding air, thus completing the translation of the electrical impulses applied tothe vibrating ring into audible sound waves.

In Fig. 5 a modied form of theV invention -is disclosed wherein an electrodynamic ring 48, surrounding a core 48a, iseonnected to asource of oscillating currentV by a pair of outwardly extending flexible conductor strips 49 and 50, said strips being preferably made integral with said ring 48. The radial strips 49 and 50 are joined to the secondary 00u51 ofA a transformer-unit 52'by 4short and ,long conducting plates 53 and 54 respectively; .The long conducting plate .54E

is adapted to'lie parallel and in close'pr'oximity,V

to the radial strips 49 and 58 Ato reduce inductive forces therebetween. As clearly shown in Fig. 5 theextremties of the strips 49 and 50 Vare oppositely offset at 55 andr andare attachedto the ends of the conductor plates :53 and 54', as by screws 51 and-58.

cation twojformsi in whichfthe inventionV may be,V embodied, it is to be understoodxtha't these formsfare'l shown for the purpose o'illustration cnly,and that the invention is not to belimited to the speciiic disclosure, but may `bemodifled and Y, Y l Y 1o While there have beendisclosed 4in this speci-v embodied in various other forms Without departing from itsspirit. In short, the inventionincludes all the modifications. and embodiments f Y coming`within the scope of therfollowing claims;

, Having. thus fully described theinvention, what.

is claimed-:aslnew and for which it isrdesired to secure LettersPatent is: f

l. In an electrodynamic acoustic vibratoigaA 2,5V

magneticfeld structure having air gaps, a closed circular. conducting ring positioned in .said air gaps, a source of oscillatory currents, and substantially non-inductive means comprising a pair of parallel flat strips adapted toicarry said curr-` y 30 rents fromsaid source to said ring. ,l

2. In an electrodynamic acoustic vibrator,2 a magnetic field structure having, air gaps,'a source of oscillatory currents, a closed vcircularfconan electrodynamic acoustic vibrator; a@

magnetic field structure including a Vcentrally apertured cylindrical core; a plurality of sta"- tionary conductors inV substantially non-inductive relationship passing through said aperture, an annular conductorcompletely surrounding said core but free to moveV axially in relation thereto,

flexible conductors within said ring adapted t0 Y support said annular conductor and to form electrical connection between the same and said stationary conductors.

bipolar eld structure having a cylindrical core and air gaps, a circular closed conducting ring ring, a transformer, and a pair of nat conductors located with their faces close together and adapta` ed to furnish electrical connection between said transformer and said flexible conducting means. 5. 'In an electrodynamic acoustic vibrator, a

4. In an electrodynamicacoustic vibrator,

field structure having two arcuate air gaps magn netically in series and geometrically opposed on a common circle, means forv magnetizing said structure transversely ofsaid circle, a circular closed conducting ring adapted to lie within saidV c air gaps and to move axially therein, exble con--V connect electrically said current source and said flexible conducting means, said air gaps extending substantially around the entire circumference of said ring.

6. In an electrodyn'amicf acoustic lvibrator, a field structure having a plurality vof arcuatel air gaps disposed about a common circle, means for magnetizing said structure, a closed circular conducting ring adapted for axial vibration within the air gaps of said structure, flexible conducting means Within and attached tosaid ring, a source of oscillating currents, and substantially non-inductive means adapted to connect electrically said current source and said flexible conducting means comprising a pair of parallel flat strips, andan acoustic diaphragm mounteduponand adapted to vibrate with said ring. "1

7. In an` electrodynamic acoustic vilcratonfaA transformer having a primary Winding, a magneticV circuit, and a secondarywindingfand adapted to function on currents having frequencies,

Within the audible range, a magnetic field, a conductor in Vthe form of a `closed circular ring. mounted for freeiaxial vibration within said mag,- netic iield, anda( pair of at, conducting; leads located with their facesfclose together.; forpro- Y viding electrical connection betweensaid trans,-l

former and diametrically opposedpoints on. said V`8. In an electrodynamic 'acoustic vibrator, a circular closed ring of conducting material having afplur'ality of inwardly extending, flexibleconductors circumferentially disposed therearound and adapted to providefaplurality ofnarcu'ate current pathselectrically in parallel in said ring, means for providing a magnetic field adapted-t0 traverse said ring, .and means for causing a flow of oscillatory currents inA said ilexible conductors and in said ring,rsaid air gaps extending substantially around the entire circumference of said ring. i i f 9'. In an Velectrodynamic Yacoustic vibrator, a eld structure having a plurality of arcuate air gaps disposed about a common circle, means for magnetizing said structure, a closed conducting ring adapted for axialvibration Withi'nthe air' gaps of 'saidfstructura ilexible conducting means within and attached to said ring, a source of oscillating currents, a pair of at insulated strips adapted to connect electrically said current source and said ilexible conducting means, and an acoustic diaphragm mounted upon and adapted toV vibrate with saidy ring. f

10. In a loudspeaker, a magnetic field energized by a constant source of direct current, an electrodynamic ring located in said eld and adapted tov vibrate `freely in a direction at right angles to said field,.a diaphragm connected to said ring, a plurality of exible current conducting strips each of .which is connected at its outer end toI said ring', means for rigidly supporting the inner ends of 'said strips, and means for passing an oscillating current through said strips and ring Vto cause Ysaid diaphragm to vibrate, said exible strips rlying in a circle concentric with said ring to minimize induction. f N

11; In an electrodynamic acoustic vibrator, a circular closed metal ring having a plurality of arcuate electrically parallel current paths, a pluralityof flexible conductors connected to said ringfat regularly spaced points, a plurality of magnetic air gaps surrounding said ring, and a pair ofV stationary conductors arranged parallel to each other for causing a ow of oscillatory currents in said conductors and in saidring.

l2. In anfelectrodynamic acoustic vibrator, a magnetic eld structure including a core, a plurality of stationary conductors 'passing through said core, anannular conductor completely surrounding said core but free'to move axially in relation thereto, inwardly extending flexible conductors adapted to support said annular conductor and to form electricalconnection between the vsame and said'stationary conductors. y

LINCOLN THOMPSON. 

